This invention relates to a polymer blend composition which is comprised of a vinylidene fluoride base copolymer and an acrylic or methacrylic ester polymer and is suitable for use as an optical material.
With the rapid advance of optical and optoelectronic devices there is an increasing demand for light transmitting plastics.
For optical fibers used in the current optical communication systems the principal materials are quartz glass and inorganic multicomponent glass. Plastics optical fibers have been developed mainly for display and short-distance transmission purposes. In general the core material of plastics optical fibers is an acrylic or methacrylic resin represented by polymethyl methacrylate and the cladding material is selected from rather special fluororesins. Besides, recently increasing attention is paid to so-called polymer cladded optical fibers using an inorganic glass as the core material and a fluororesin as the cladding material.
Advantages of plastics optical fibers reside in good flexibility or pliability and ease of fabrication and processing. Since conventional plastics optical fibers are larger in transmission loss than inorganic glass optical fibers, still there is a keen demand for Plastics having improved transparency and refractive characteristics.
Thus far, optical fibers using plastics are mostly of the step-index type. At present efforts are devoted also to the development of optical plastic materials of the graded-index type for use not only in optical fibers but also in other optical elements such as cylindrical lenses. As optical materials, fluororesins are highly valued because of their inherently low refractivity by the effect of the atomic refraction of fluorine. One way to desirably control the refractive index of plastics is blending two kinds of polymers or copolymers different in refractivity. However, not many kinds of practicable optical materials have been obtained by this technique. The main reason for low availability of this method is because two kinds of polymers having very different indices of refraction are in most cases poor in compatibility or mutual solubility so that blend of such two polymers becomes low in mechanical strength and also in transparency.
According to JP-A No. 59-41348, a polymer blend of polymethyl methacrylate, or a copolymer of methyl methacrylate and another methacrylate or acrylate, and a copolymer of vinylidene fluoride and tetrafluoroethylene is good in mutual solubility and serves for optical purposes. According to JP-A No. 59-62657, similarly good mutual solubility is exhibited also when a copolymer of vinylidene fluoride and trifluoroethylene is used in place of the copolymer in JP-A No. 59-41348. However, these polymer blends are not fully satisfactory in transparency and are liable to partially crystallize at elevated temperatures. U.S. Pat. Nos. 4,591,616 and 4,617,350 show polymer blends of a binary copolymer of vinylidene fluoride and hexafluoroacetone with a methyl methacrylate base polymer or an acrylate base polymer. As disclosed in these patents, mutual solubility between a vinylidene fluoride base polymer and an acrylate or methacrylate base polymer is improved by incorporating hexafluoroacetone in the former polymer, and the polymer blends of these patents are improved in transparency too. However, still there is a demand for plastics having more improved transparency together with other properties required for optical materials.